But seriously, it might be a better solution to just get a couple of boxes of ammo for you 308 and head to the range for a bit and get comfortable with the scope. You already know that what you have will work so teach him to shoot it and don't worry about learning how to make it work with a shotgun.

I have hunted my whole life with a shotgun. I would use an IC choke tube and Breneke slugs or Rem Sluggers. Any Foster type slug should work in a IC choke tube from 0 to 75 yds.. You can also go with a rifled choke tube and try the Lightfield or any sabot type slug. A good set of fiber optic sights mounted on the vent rib will help also and you can adjust them to POI. Hope this helps !!!